m gauge like a thermometer, with a
flattened bulb; and the pressure of the steam, by compressing the bulb,
causes the mercury to rise to a point proportional to the pressure applied.

THE INDICATOR.

234. _Q._--You have already stated that the actual power of an engine is
ascertained by an instrument called the indicator, which consists of a
small cylinder with a piston moving against a spring, and compressing it to
an extent answerable to the pressure of the steam. Will you explain further
the structure and mode of using that instrument?

[Illustration: Fig. 36]

_A._--The structure of the common form of indicator will be most readily
apprehended by a reference to fig. 36, which is a McNaught's indicator.
Upon a movable barrel A, a piece of paper is wound, the ends of which are
secured by the slight brass clamps shown in the drawing. The barrel is
supported by the bracket _b_, proceeding from the body of the indicator,
and at the bottom of the barrel a watch spring is coiled with one end
attached to the barrel and the other end to the bracket, so that when the
barrel is drawn round by a string wound upon its lower end like a roller
blind, the spring returns the barrel to its original position, when the
string is relaxed. The string is attached to some suitable part of the
engine, and at every stroke the string is drawn out, turning round the
barrel, and the barrel is returned again by the spring on the return
stroke.

235. _Q_--But in what way can these reciprocations of the barrel determine
the power of the engine?

_A._--They do not determine it of themselves, but are only part of the
operation. In the inside of the cylinder _c_ there is a small piston moving
steam tight in a cylinder of which _d_ is the piston rod, and _e_ a spiral
spring of steel, which the piston, when forced upwards by the steam or
sucked downwards by the vacuum, either compresses or extends; _f_ is a cock
attached to the cylinder of the indicator, and which is screwed into the
cylinder cover. It is obvious that, so soon as this cock is opened, the
piston will be forced up when the space above the piston of the engine is
opened to the boiler, and sucked down when that space is opened to the
condenser--in each case to an extent proportionate to the pressure of the
steam or the perfection of the vacuum, the top of the piston _c_ being open
to the atmosphere. A pencil, _p_, with a knife hinge, is inserted into the
piston rod, at _e_, and t